This application is based upon and claims the benefit of priority of Japanese Patent Application No. 2000-139864 filed on May 12, 2000, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a sensor having a membrane structure.
2. Description of the Related Art
In recent years, a thermopile-type infrared sensor is manufactured as a small-sized one having a membrane by using semiconductor-material micro-machining techniques. For example, xe2x80x9cSensor Technologyxe2x80x9d, Vol. 2, No. 4, April 1982 discloses that sensitivity of the sensor having such structure is improved by sealing a sensor chip (device) under vacuum. Further, JP-A-6-74818 discloses that sensitivity of the sensor is improved by sealing a sensor chip in low thermal-conductivity gas such as xenon gas.
The sensor chip can be sealed under vacuum or specific gas in the following steps. Specifically, first, the sensor chip is bonded to a stem, and wire bonding is carried out. Thereafter, a cap is welded under vacuum or the specific gas, so that the sensor chip bonded to the stem is sealed.
However, a through hole exists under the membrane and the through hole is sealed when the sensor chip is bonded to the stem. Therefore, when the sensor chip, bonded to the stem, is sealed under vacuum or the specific gas, gas within the through hole remains as it is. As a result, the following troubles are generated. In an infrared sensor or the like, a membrane is formed from a thin film for lowering thermal conductivity. Therefore, when the space surrounding the sensor chip bonded to the stem is evacuated in state where the inside of the through hole has atmospheric pressure, the membrane may be broken due to pressure difference.
Further, even when the sensor chip is used in atmospheric air without being sealed in the specific gas, if the through hole is hermetically sealed, gas within the through hole expands and contracts thermally and repeatedly due to changes in temperature. As a result, the membrane is repeatedly bent (distorted), so that sensor characteristics is changed or the sensor is broken in extreme cases.
The present invention has been made in view of the above problems. An object of the present invention is to provide a sensor having a membrane structure with high reliability.
According to the present invention, briefly, a communication passage is provided at a joint portion between a semiconductor substrate and a base so that an inside and an outside of a through hole in the semiconductor substrate communicate with each other through the communication passage. Accordingly, the inside and the outside of the through hole can have the same pressure. That is, an upper surface and a lower surface of a membrane structure closing the through hole at an opposite side of the base can be subject to the same pressure. As a result, the membrane structure is prevented from being damaged, resulting in high reliability.